Psychological Bullet in1982, Vol. 91, No. 1, 276-292

Copyright 1982 by the American Psychological Association, Inc.0033-2909/82/9102-0276S00.75

Task-Evoked Pupillary Responses, Processing Load,and the Structure of Processing Resources

Jackson BeattyDepartment of Psychology and Brain Research Institute

University of California, Los Angeles

A physiological measure of processing load or "mental effort" required to performa cognitive task should accurately reflect within-task, between-task, and between-individual variations in processing demands. This article reviews all availableexperimental data and concludes that the task-evoked pupillary response fulfillsthese criteria. Alternative explanations are considered and rejected. Some im-plications for neurophysiological and cognitive theories of processing resourcesare discussed.

That the pupil of the eye dilates duringmental activity has long been known in neu-rophysiology. For example, Bumke, the Ger-man neurologist, wrote seven decades ago(as translated in Hess, 1975):Every active intellectual process, every psychical effort,every exertion of attention, every active mental image,regardless of content, particularly every affect just astruly produces pupil enlargement as does every sensorystimulus, (pp. 23-24)

Only recently has this phenomenon beenused as a tool in investigating human cog-nitive processing. The pupillary dilationsthat accompany cognitive processes are in-deed as pervasive a phenomenon as Bumkehad indicated. They occur at short latenciesfollowing the onset of processing and subsidequickly once processing is terminated. Per-haps most important, the magnitude of pu-pillary dilation appears to be a function ofprocessing load or "mental effort" requiredto perform the cognitive task.

These facts led Kahneman (1973) to relyon the task-evoked pupillary response as theprimary measure of processing load in hiseffort theory of attention. He justified theuse of this physiological measure in termsof the strong empirical relation between task

This research was sponsored by Office of Naval Re-search Contract N00014-76-C-061G.

I thank Raja Parasuraman for his critical commentsduring the writing of the manuscript.

Requests for reprints should be sent to JacksonBeatty, Department of Psychology, University of Cali-fornia, Los Angeles, California 90024.

demands and pupillary dilation, leading tothe conclusion that "the limited capacity andthe arousal system must be closely related"(p. 10). Kahneman proposed three criteriafor any physiological indicator of processingload: It should be sensitive to within-taskvariations in task demands produced bychanges in task parameters; it should reflectbetween-task differences in processing loadelicited by qualitatively different cognitiveoperations; finally, it should capture be-tween-individual differences in processingload as individuals of different abilities per-form a fixed set of cognitive operations. Thefirst section of this article reviews the evi-dence that the task-evoked pupillary re-sponse may serve as such an indicator.

Task-Evoked Pupillary Responses as aMeasure of Processing Load

Changes in central nervous system activitythat are systematically related to cognitiveprocessing may be extracted from the rawpupillary record by performing time-lockedaveraging with respect to critical events inthe information-processing task. A task-evoked pupillary response bears the samerelation to the pupillary record from whichit is derived as does an event-related brainpotential to spontaneous electroencephalo-graphic activity. With averaging, short-la-tency (onset between 100 and 200 msec),phasic task-evoked dilations appear, whichterminate rapidly following the completionof processing.

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Figure I . Task-evoked pupillary responses in threeshort-term memory tasks. [(A) Digit span for stringlengths of three through seven digits. Time axes havebeen adjusted so that the 2-sec pause between presen-tation and recall is superimposed for all lengths. Slashmarks are placed on each curve just before the first digitis presented and after the last digit is repeated. Theamplitude of the response grows during item presenta-tion, reaching a maximum during the pause and returnsto baseline during report. Peak amplitude is a monotonicfunction of the number of items held in memory.(Adapted from "Pupil diameter and load on memory"by D. Kahneman & J. Beatty, Science, 1966,154, 1583-1585. Copyright 1966 by the American Association for

Within-Task Variations inProcessing Load

In the last two decades, task-evoked pu-pillary responses have been obtained for awide variety of cognitive processes, rangingfrom sensory detection through memory,language processing, attention, and complexreasoning. The evidence relating to the cor-respondence between processing demandsand pupillary response within individualtasks is as follows.

Short-term memory. The study of short-term memory formed an initial and enduringproblem in the pupillometric investigationof information processing. Kahneman andBeatty (1966) presented the first pupillo-metric analysis of task-evoked pupillary re-sponses in a short-term memory task (seeFigure 1A). Strings of three-seven digitswere aurally presented at the rate of 1 persec. After a 2-sec pause, subjects were re-quired to repeat the digit string at the samerate. Under these conditions, pupillary di-ameter increases with the presentation ofeach digit, reaching a maximum in the pausepreceding report. During report, pupillarydiameter decreases with each digit spoken,reaching baseline levels after the final digit.The magnitude of the peak pupillary dilationin this task is an increasing function of stringlength. Kahneman and Beatty (1966) alsoobserved that if the subject were requestedto repeat the string a second time immedi-ately after reporting the final digit, the pupilimmediately redilates to the peak diameterfor that string and then decreases with eachdigit spoken until the entire string has been

the Advancement of Science. Reprinted by permission.)(B) Responses for four digits, four words, and a four-digit transformation task. The slope of the pupillaryresponse is a function of item difficulty. (Adapted from"Pupil diameter and load on memory" by D. Kahneman& J. Beatty, Science, 1966, 154, 1583-1585. Copyright1966 by the American Association for the Advancementof Science. Reprinted by permission.) (C) Pupillary re-sponses during five-, nine-, and 13-digit test and controltrials. Pupillary diameter increases until approximatelyseven items are held in memory, after which the curveof the response becomes asymptotic. (Adapted from"Individual differences in pupil size and performance"by S. Peavler, in Pupillary dynamics and behavior byM. Janisse (Ed.), p. 164. Copyright 1974 by PlenumPress. Reprinted by permission.)]

278 JACKSON BEATTY

reported for the second time. Beatty andKahneman (1966) demonstrated that a sim-ilar pupillary function is obtained when astring of items is recalled from long-termmemory: On request to report, a large pu-pillary dilation is observed as information isretrieved from long-term memory and theresponse is organized. As each digit in thestring is spoken, pupillary diameter de-creases, reaching baseline levels after thelast digit is spoken.

The slope of this task-evoked pupillaryresponse is determined by the difficulty ofthe to-be-remembered information as in-dexed by memory span for different typesof items. Kahneman and Beatty (1966)tested three conditions: recall of four digits,recall of four unrelated nouns, and trans-formation of a four-digit string by addingone to each item (see Figure IB). The slopeof the pupillary response during input wassmallest for the least difficult items, thestrings of four digits that were to be simplyrepeated. A steeper slope was observed forthe strings of four words. The greatest slopewas obtained for the most difficult task, digitstring transformation. Thus, both item dif-ficulty and number of items affect the pu-pillary response in short-term memory tasks.

The idea that the task-evoked pupillaryresponse provides a physiological measureof processing load received direct support ina subsequent experiment by Kahneman,Beatty, and Pollack (1967), in which bothpupillometric and behavioral interferencemethods were used to assess processing loadin the four-digit add-one memory transfor-mation task. In using a secondary task ofvisual target detection, it was found that theamplitude of the task-evoked pupillary re-sponse was a reliable predictor of load-in-duced performance decrements in the sec-ondary task. A series of controls ruled outany peripheral interference of the pupillarydilations themselves on performance of thesecondary task.

Rehearsal strategies that improve perfor-mance on a short-term memory task act toreduce the amplitude of the task-evoked pu-pillary response. Kahneman, Onuska, andWolman (1968) presented subjects withstrings of nine digits, either at a uniform rateof 1 per sec or with a temporally imposed

three-digit grouping (.5 sec separating digitswithin a group and 2.0 sec separatinggroups). The grouped mode of presentationhad previously been shown to increase digitspan materially (Ryan, 1967), presumablyby breaking the string into more codableunits or chunks (Miller, 1956). The pupil-lometric data reflected the experimentallyinduced differences in processing strategy:A steady monotonic increase in pupillarydiameter accompanied presentation of thedigits at the uniform rate, whereas waves ofdilation during presentation and constrictionduring the intergroup pauses characterizedthe grouped presentation condition. Thus,the task-evoked pupillary response appearsto reflect changes in information-processingdemands induced by processing strategiesthat affect performance.

The idea that the pupillary response mea-sures processing load found further supportin Peavler's (1974) study of informationoverload (see Figure 1C). The capacity ofshort-term memory for strings of unrelateddigits is approximately 7 (Miller, 1956).Peavler measured the task-evoked pupillaryresponse for strings of five, nine, and 13 dig-its, which were randomly intermixed in pre-sentation. During presentation of the strings,pupillary diameter increased as an increas-ing function of memory load for digits 1through 7. At the seventh or eighth digit, thepupillary response reached an asymptote; nofurther dilation was observed. These datasuggest that as long as some information-processing capacity remains, increasingmemory load is reflected by increasing pu-pillary dilation. Once the limits of capacityare exceeded, however, further increases intask demands no longer yield increased pu-pillary dilation.

Language processing. Several aspects oflanguage processing have been studied pu-pillometrically. At the most molecular level,Beatty and Wagoner (1978) used an exper-imental method developed by Posner (Posner& Boies, 1971; Posner & Mitchell, 1967) tostudy the visual encoding of single letters.In Beatty and Wagoner's first experiment,subjects were required to judge whether ornot a pair of visually presented letters hadthe same name. Individual letters were pre-sented in either upper or lower case type.

PUPILLARY RESPONSES AND PROCESSING LOAD 279

Thus, two kinds of letter pairs could bejudged to be the same by the name criterion.If both letters are presented in the same case(e.g., AA or aa), only the physical featuresof the letters need to be analyzed to reachthe correct judgment. If they differ in case(e.g., Aa or bB), then, in addition to ana-lyzing the physical features, a second processof name code extraction must be performed.Figure 2A presents these data. Although thetask-evoked pupillary responses were smallin this simple task (on the order of . 1 mm),they did reflect the extra processing requiredfor name code extraction. Significantly largerresponses were obtained for letter pairs thatdiffered in case.

In a second similar experiment, Beatty

and Wagoner (1978) examined three levelsof character encoding by requiring the useof a higher order category classification(vowels and consonants). Thus, letter pairscould be physically identical, identical inname, or identical in category membership(e.g., Ae or BK). Again, the task-evokedpupillary response reflected the processingrequired to perform the letter-matching taskat each level (see Figure 2B).

Ahern (Ahern, 1978; Ahern & Beatty,1981) undertook two experimental investi-gations involving language processing aspart of a larger research program on indi-vidual differences in intelligence. The firstof these experiments examined task-evokedpupillary responses in the perception and

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Figure 2. Task-evoked pupillary responses in the Posner letter-matching task. [(A) Responses for cor-rectly identified same and different letter pairs using a name rule. The responses to same judgments arelarger when name code extraction is required. In control trials, subjects always saw the letter pair XXand were required to respond "same." (B) Responses in the letter-matching task using a category rule.Again, the amplitude of the responses for same judgments increases with complexity of processingrequired to reach that judgment. (Adapted from "Pupillometric signs of brain activation vary with levelof cognitive processing" by J. Beatty & B. L. Wagoner, Science, 1978, 199, 1216-1218. Copyright 1978by the American Association for the Advancement of Science. Reprinted by permission.)]

280 JACKSON BEATTY

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Figure 3. Task-evoked pupillary responses for four levelsof sentence complexity in Baddeley's Grammatical Rea-soning Task. [The amplitude of the response is signif-icantly greater for. the longer, syntactically more com-plex sentences. (Adapted from Ahern, 1978).]

comprehension of words. Subjects were re-quired to judge pairs of words as similar ordifferent in meaning. The first word of eachpair was drawn from either the easiest or themost difficult items of one of three psycho-metric vocabulary tests. The second word,presented 2 sec later, was either a synonymof the first or quite different in meaning. Inthis experiment, the ease of retrieving lexicalinformation was reflected in the pupillaryresponse. A dilation of approximately .1 mmfollowed the presentation of the easy targetwords, whereas the dilation for the difficulttarget words was twice as large. A seconddilation followed the presentation of thecomparison word, yielding pupillary dila-tions of .30 and .35 mm, respectively, duringthe judgment period. Thus, larger pupillarydilations accompany the semantic processingof psychometrically more difficult vocabu-lary items.

At the most molar level, task-evoked pu-pillary responses have been studied as sub-jects processed meaningful sentences of dif-ferent complexity. Ahern (Ahern, 1978;Ahern & Beatty, 1981), using Baddeley'sGrammatical Reasoning Task (Baddeley,1968), presented sentences of the form "Afollows B" or "B precedes A," after whichan exemplar "AB" or "BA" was given. Thetask was to determine whether the sentencecorrectly described the exemplar. Sentences

differed in grammatical complexity, beingactive-positive, active-negative, passive-positive, or passive-negative. Although thesesentences differed in length, sentence dura-tion was held constant by using computerpresentation of digitized natural speech. Inthis task, increasing dilation was observedduring the presentation of the sentence andthe exemplar, which peaked during the de-cision interval (see Figure 3). The amplitudeof these responses averaged approximately.40 mm and differed significantly as a func-tion of grammatical complexity, with thelonger, more complex sentences elicitinglarger pupillary responses.

Wright and Kahneman (1971) also ap-plied pupillometric measurements in a sen-tence-processing task. Subjects were pre-sented with complex sentences of the form"The qualified managing director was re-cently sensibly appointed by the expandingsuccessful company." Subjects were re-quired either to repeat the sentence or toanswer a question of the form "Who ap-pointed the director in this sentence?" Thequery was posed either before or after thesentence was presented. When the task wasto repeat the sentence, the task-evoked pu-pillary response increased as the sentencewas presented and peaked during the reten-tion interval (3 or 7 sec), reaching a maxi-mum dilation of approximately .30 mm.When the question was posed after thepause, peak dilation during the pause wasapproximately .20 mm and was followed byanother dilation as the answer to the ques-tion was formed. The peak of this dilationwas approximately .40 mm with respect topre-sentence baseline. When the questionwas posed before sentence presentation, thetask-evoked pupillary response rose moregradually but increased rapidly when therelevant portion of the sentence was pre-sented, indicating organization and process-ing of the answer to the query. No evidenceof processing of phrase boundaries was ob-served, but, as Wright and Kahneman com-mented, their sentences were not represen-tative of those naturally occurring in spokenEnglish.

Beatty and Schluroff (Note 1) studied theeffects of both syntactic and semantic or-ganization on the task-evoked pupillary re-

PUPILLARY RESPONSES AND PROCESSING LOAD 281

sponse and performance in the encoding andreproduction of six-word sentences, A baseset of 12 standard sentences using one of sixsyntactic constructions was used to test per-formance with both syntactic and semanticorganization. An example of such a sentenceis "Should blind people lead quiet lives?" Ina second condition, the semantic organiza-tion of these sentences was reduced by ex-changing words between sentences whilemaintaining the syntactic frame (e.g., "Manyblind roses play heavy trouble")- In the thirdcondition syntactic organization was alsoeliminated by selecting random strings ofitems (e.g., "Rains children milk golden usu-ally medals"). Figure 4 presents the task-evoked pupillary responses obtained underthese conditions. First, on each of the rec-ords, small dilations may be observed duringthe reception and production of each of theindividual words in the string. Second, bothsyntactic and semantic organization actedto reduce the amplitude of the task-evokedpupillary response. Third, these effects oflinguistic organization were present at input

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and during the pause preceding report: Noadditional effect of linguistic organizationwas observed during production. Finally,Beatty and Schluroff conducted a secondaryanalysis for the two conditions with syntacticorganization in which sentence frames oflower and higher complexity were compared.The more complex syntactic frames yieldedsignificantly larger task-evoked pupillary re-sponses in both normal and semanticallyanomalous sentences.

Reasoning, Mental arithmetic has beenused as an example of a complex reasoningproblem by several investigators. Hess andPolt (1964), in their initial and influentialarticle on pupillary signs of mental activity,measured pupillary diameter as five subjectssolved four multiplication problems, rangingin difficulty from 7 X 8 to 16 X 23. For eachof the subjects and each of the problems,pupillary diameter increased from the mo-ment of problem presentation until the pointof solution. Hess and Polt reported thesedata as percentage dilation, not as absolutevalues. Across subjects, the percentage di-

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Figure 4. Task-evoked pupillary responses for six-word sentences differing in linguistic organization.[Standard sentences were meaningful English sentences. Anomalous sentences used the same syntacticframes but with words interchanged between sentences to render the strings nearly meaningless. Scram-bled sentences had neither syntactic nor semantic organization. Both syntactic and semantic organizationindependently reduced the processing load imposed by the sentence repetition task. Open arrows indicatepresentation of words and the response cue; filled arrows indicate timing clicks. (From Beatty & Schluroff,Note 1).]

282 JACKSON BEATTY

lation was perfectly ordered by difficulty ofthe problem.

Bradshaw (1968) reported similar resultsfor six subjects performing mental divisionproblems at two levels of difficulty. Pupillarydiameter increased during problem solvinguntil the point of solution, peak dilationbeing larger for the more difficult problems.Similarly, Payne, Parry, and Harasymiw(1968) described a monotonic relation be-tween mean pupillary diameter and problemdifficulty but noted that this relationship ismarkedly nonlinear with respect to difficultyscales based on percent correct solution, timeto solution, or subjective rating of difficulty.Pupillary diameter in mental multiplicationappears to peak rapidly as a function of dif-ficulty, with more difficult problems requir-ing more time until solution is reached.

These results were subsequently replicatedby Ahern and Beatty (1979, 1981). Threelevels of problem difficulty were used, rang-ing from multiplying pairs of one-digit num-bers to multiplying pairs of two-digit num-bers ranging between 11 and 20. Figure 5presents these data. In this task an initialdilation of approximately .15 mm accom-panies the encoding and storage of the mul-tiplicand. The second and major dilation fol-lows presentation of the multiplier andcontinues through problem solution. Boththe amplitude and latency of this latter di-lation increase as a function of problem dif-ficulty. In the most difficult condition, theresponse reached an asymptote at approxi-mately .50 mm.

Perception. Small but reliable pupillarydilations accompany the detection of bothvisual and acoustic signals at near-thresholdintensities. Hakerem and Sutton (1966) pro-vided the first pupillometric analysis of pro-cessing load in perceptual detection. Sub-jects viewed a uniform visual field on whichbrief increments in luminance could be im-posed as pupillary diameter was measured.When the magnitude of the intensity incre-ment was adjusted to yield 50% correct de-tection, all vestiges of the flash-induced lightreflex disappeared. Under these conditions,a clear pupillary dilation of approximately.10 mm was observed if, and only if, a pre-sented target was detected. Figure 6A pre-sents these data.

Beatty and Wagoner (Note 2) extendedHakerem and Sutton's (1966) finding to au-dition, using weak 100 msec 1 kHz sinu-soidal acoustic signals presented against abackground of white noise. Signals were pre-sented on each trial with a probability of .50.After each trial, the subjects rated their cer-tainty that a target had or had not been pre-sented (Green & Swets, 1966). For signal-present trials, the magnitude of the task-evoked pupillary response was largest forsignals judged with high certainty to be pres-ent and smallest for signals judged with highcertainty to be absent. Amplitudes for un-certain judgments assumed intermediatevalues (see Figure 6B). These results fullyconfirm those reported by Hakerem and Sut-ton for visual detection.

It is of interest that the signal detectiontask provides one instance in which increas-ing task difficulty does not increase the am-plitude of the pupillary response. Beatty andParasuraman (Note 3) reported that manip-ulation of the signal-to-noise ratio of thestimulus affects the performance but not theamplitude of the task-evoked pupillary re-sponse. They interpret this finding as furtherevidence that acoustic signal detection is adata-limited, not a resource-limited, process(Norman & Bobrow, 1975). For a data-lim-ited process, performance quality is deter-mined by input data quality; there are noadditional processing steps or proceduresthat the listener may use to increase furtherhis or her ability to detect the signal. Ittherefore follows that decreasing the detect-ability of a weak sensory signal aversely af-fects performance without increasing theprocessing load associated with the task.

Task-evoked pupillary responses are alsoobserved in perceptual discrimination tasks,in which a presented stimulus must be com-pared against memory and a judgment ren-dered. Kahneman and Beatty (1967) re-ported the first study of the pupillary responsein perceptual discrimination. On each trialthe subject heard a standard tone of 850 Hz,which was followed 4 sec later by a com-parison tone. The comparison was one of 11frequencies, ranging between 820 and 880Hz in 6-Hz steps. The subject's task was tojudge whether the comparison tone washigher or lower in pitch than the standard

PUPILLARY RESPONSES AND PROCESSING LOAD 283

MULTIPLICAND MULTIPLIER

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Figure 5. Task-evoked pupillary responses in mentalmultiplication. [There is an initial dilation during theencoding of the multiplicand followed by a substantialdilation on presentation of the multiplier and the be-ginning of problem solving. Both the amplitude and thepeak latency of this major dilation increase as a functionof problem difficulty. (From Ahern, 1978).]

dropped as a function of time over the 48min of the task (from 84% to 67%). Theamplitude of the task-evoked pupillary re-sponse showed a similar reduction, from ap-

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tone. The difficulty of the discrimination isinversely related to the difference in pitchbetween the comparison and the standard.The amplitude of the response to the com-parison tone varied as a function of discrim-ination difficulty, from approximately .10mm for easy to .20 mm for difficult discrim-inations.

Sustained attention. Processing effi-ciency in memory-dependent perceptual dis-crimination tasks is known to deteriorate ifthe task is prolonged and the number of dis-criminations required per minute is rela-tively high (Parasuraman, 1979; Parasura-man & Davies, 1977). One theory to explainthis vigilance decrement is that central ner-vous system activation deteriorates over timeunder such conditions, and as a result theadequacy of information processing is in-creasingly compromised. Such changes mightappear in either tonic or phasic pupillometricmeasures. Thus, Beatty (in press) measuredtask-evoked pupillary responses to nontargetstimuli in an auditory vigilance task. Non-target stimuli were 50-msec 1-kHz tonebursts, presented at intervals of 3.2 sec. Ran-domly intermixed were target stimuli, whichwere attenuated by 3.5 db. Subjects reportedthe detection of targets by depressing a mi-croswitch. Under these conditions, the effi-ciency of target/nontarget discrimination

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Figure 6. Task-evoked pupillary responses in signal de-tection tasks. [(A) Visual signal detection. A clear .1-mm dilation is observed following detection of a signalthat is not present for undetected signals or blank trials.(From "Pupillary response at visual threshold" by G.Hakerem & S. Button, Nature, 1966, 2/2(5061), 485-486. Copyright 1966 by Macmillan Journals Limited.Reprinted by permission.) (B) Auditory signal detec-tion. When a rating response (YC = yes, certain; YU= yes, uncertain; NU = no, uncertain; NC = no, certain)is used, the pupillary response increases with the judgedlikelihood that a signal was presented on that trial.(From Beatty & Wagoner, Note 2).]

284 JACKSON BEATTY

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Figure 7. Task-evoked pupillary responses in auditoryselective attention. [Small (.01 mm) dilations may beobserved following presentation of background stimulion the attended but not on the unattended auditorychannel. (From Beatty-, Note 4).]

proximately .07 mm in the first third of thetask to .04 mm in the last. Tonic or baselinepupillary diameter exhibited no such relationwith performance. Thus, the physiologicalmechanisms operating during informationprocessing under alerted conditions appearto be altered under conditions eliciting a vig-ilance decrement.

Selective attention. Selective attentionalprocessing of sensory information occurs un-der conditions of high information load whenit is not possible to process adequately allincoming information. A commonly citedexample of selection in linguistic processingis the cocktail party phenomenon, in whichthe listener selects one voice among manyto be attended to and processed. Electro-physiological evidence of selective atten-tional processes has been obtained by Hill-yard and his co-workers (Hillyard, Hink,Schwent, & Picton, 1973) using a multiplechannel tone discrimination task. Beatty(Note 4) used this procedure to test for theeffects of selective attention in the task-

evoked pupillary response. Subjects were re-quired to listen to a series of randomly pre-sented high- and low-frequency tones pre-sented at an average rate of 3 per sec. Thesubject's task was to attend to one of the twotypes of tones and to press a switch whenevera target tone (marked by a slight frequencyincrement) occurred. Under these conditionsa small (.015 mm) pupillary dilation at alatency of 600 msec followed presentationof nontarget tones on the attended channel,which was completely absent following stim-uli on the nonattended channel. Figure 7presents these data. Although the amplitudeof these responses was extremely small, thedifferences between attended and nonat-tended tones was highly significant.

Between-Task Variations inProcessing Load

Kahneman's (1973) second criterion fora physiological measure of mental effort isthat the measure should order variations inprocessing demands across qualitatively dif-ferent mental tasks. In each of the experi-ments described in the preceding section,there appears to be an orderly relationshipbetween the processing demands imposed bya cognitive task and the amplitude of thetask-evoked pupillary response. Moreover,tasks that place large demands on the in-formation-processing system—judged be-haviorally, subjectively, or by an analysis oftask requirements—elicit large task-evokedpupillary responses. Less demanding taskselicit smaller responses. It is possible, there-fore, that task-evoked pupillary responsesassociated with cognitive function mightprovide a common metric for the assessmentand comparison of information-processingload in tasks that differ substantially in theirfunctional characteristics. Underlying thisproposal is the idea that the dynamic changesindexed by the task-evoked pupillary re-sponse reflect a basic physiological aspect ofprocessing load that is independent of qual-itative differences between tasks.

The usefulness of such intertask compar-isons is strengthened by the finding that themagnitude of the task-evoked pupillary re-sponses during cognitive processing is inde-pendent of baseline pupillary diameter over

PUPILLARY RESPONSES AND PROCESSING LOAD 285

a physiologically reasonable but not extremerange of values (Bradshaw, 1969, 1970;Kahneman & Beatty, 1967; Kahneman etal., 1967). It is therefore possible to comparethe absolute values of the task-evoked dila-tions reported from different laboratories forqualitatively different tasks. Figure 8 pre-sents such a quantitative comparison, givingthe approximate peak amplitude of the task-evoked pupillary response measured frompublished figures for each of the tasks de-tailed above, subject only to the constraintthat the data are not confounded by the ef-fects of overt response.

The leftmost panel of Figure 8 presentspeak dilations for short-term memory tasks.The data for short-term retention of digitsare the average of the values obtained byAhern (1978), Kahneman and Beatty (1966),Kahneman, Onuska, and Wolman (1968),and Peavler (1974). The value for retentionof four words is from Kahneman and Beatty.The next panel summarizes the literature onlanguage processing. The peak value for theletter-matching task (Posner & Mitchell,1967) is the average of both experimentspublished by Beatty and Wagoner (1978).Sentence encode-1 is from Wright and

Kahneman (1971). Sentence encode-2 isfrom Beatty and Schluroff (Note 1). Allother values for language-processing tasksare taken from Ahern. Word encoding is theresponse to the presentation of the first wordin the synonyms judgment task. The valuesfor easy and difficult word matching are thepeak response during the judgment periodfollowing presentation of the second word inthat task. The value for grammatical rea-soning is the average of the four types ofsentences in Baddeley's Grammatical Rea-soning Task (Baddeley, 1968).

The third panel presents data from themental multiplication task used as an ex-ample of complex reasoning. Only Ahern(1978) presented task-evoked pupillary re-sponses for this task that are necessary forcomparative peak measurement. Multipli-cand storage is the amplitude of the peakresponse to the first item in the multiplica-tion task. The other three values are the peakamplitudes attained during problem solu-tion.

The rightmost panel presents data for per-ceptual tasks. The visual detection data arefrom Hakerem and Sutton (1966), and theauditory detection data are from Beatty and

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Figure 8. Peak amplitudes of the task-evoked pupillary responses obtained in a range of qualitativelydifferent cognitive tasks, arranged by type of task. (The pupillary response provides a reasonable orderingof tasks on the basis of presumed processing load. See text for further details.)

286 JACKSON BEATTY

Wagoner (Note 2). The discrimination dataare taken from Kahneman and Beatty(1967).

Several points concerning these data de-serve mention. First, the data are tolerantof the stringent demands placed on them incomparing absolute dilation values acrossexperiments. Rescaling of some sort is oftenrequired for physiological data to removeindividual differences in responsivity (John-son & Lubin, 1972). No such rescaling wasundertaken here. The data plotted are ab-solute peak dilations obtained from differentgroups of subjects performing a range ofcognitive tasks under varying experimentalconditions in different laboratories. Second,the data plotted in Figure 8 are internallyconsistent. No abnormally large or smallvalues are present. Third, the ordering ofthese values corresponds quite closely to anordering of these tasks using other criteriaof information-processing load. The short-term memory tasks cover a large range ofvalues, depending on the number of itemsheld for recall. Similarly in language pro-cessing, the sentence comprehension tasksyield large pupillary dilations, whereas thesimpler word- and letter-matching taskselicit much smaller values. The mental mul-tiplication tasks again span a wide range ofvalues, each appropriate to the difficulty ofthe particular problem. Finally, the percep-tual tasks, which behavioral techniques in-dicate impose minimal processing load, areassociated with small task-evoked pupillaryresponses (Wickens, Note 5).

Thus, Figure 8 provides evidence that thetask-evoked pupillary responses faithfullyreflect variations in processing load betweenqualitatively different cognitive tasks. Infact, this physiological phenomenon providesa primary reason for retaining some form ofa general metric of processing load, an ideathat has recently come under attack becauseof a failure of simple general capacity mod-els to predict adequately two-task interac-tions when time sharing (Navon & Gopher,1979). Thus, the task-evoked pupillary re-sponse seems to fulfill Kahneman's secondcriterion of measuring variations of process-ing load between qualitatively different men-tal tasks.

Between-Individual Variations inProcessing Load

The only published test of task-evokedpupillary responses as an index of between-subject variations of processing load imposedby a cognitive task was provided by Ahemand Beatty (1979, 1981). They measuredtask-evoked pupillary responses in two groupsof university undergraduates who differedin psychometrically measured intelligence.Subjects were selected on the basis of com-bined verbal and quantitative ScholasticAptitude Test scores, being either under 950or above 1350 for the low- and high-intel-ligence groups, respectively. Four cognitivetasks were used: mental multiplication, digitspan, vocabulary information, and sentencecomprehension. In each of these tasks, atleast two levels of task difficulty were used,and in all cases the more difficult task pa-rameters elicited larger task-evoked pupil-lary responses. Further, all tasks were sen-sitive to the between-group differences inputative intelligence; in each task, subjectsin the high-intelligence group made fewererrors.

For three of the four tasks, significant be-tween-group differences in the amplitude ofthe task-evoked pupillary response were ob-served. Figure 9 presents the task-evokedpupillary responses for the mental arithmetictask. With the exception of the vocabularytask, in which the pupillary responses wereessentially identical in both groups, the task-evoked pupillary response amplitudes wereconsistently smaller for the more intelligentsubjects than for their less intelligent coun-terparts. These between-group differenceswere interpreted as indicating that perfor-mance of the same objective cognitive tasksis less demanding for more intelligent indi-viduals. In addition, the amplitudes of theautonomically mediated light and dark re-flexes were measured to test for possible con-founding with group differences in auto-nomic excitability. The reflex responses,however, were identical in the two groups,suggesting that the observed between-indi-vidual differences in the amplitude of thetask-evoked pupillary response reflect cen-tral rather than peripheral aspects of neural

PUPILLARY RESPONSES AND PROCESSING LOAD 287

function. These data suggest that the task-evoked pupillary response might fulfillKahneman's third requirement for a mea-sure of processing load, that the index wouldreflect between-individual differences as wellas task differences.

Alternative Interpretationsof the Task-Evoked Pupillary Response

The definition of the task-evoked pupillaryresponse as a measure of processing loaddepends not only on the clear demonstrationthat the response varies with relevant taskparameters but also on evidence excludingthe involvement of other potentially con-founding variables. In this context, it is im-portant to distinguish between factors af-fecting the task-evoked pupillary responseand those that affect tonic or baseline pu-pillary diameter. Basal diameter is stronglyinfluenced by a wide variety of systemic andenvironmental factors (Lowenstein & Low-enfeld, 1962). The procedure of averagingphasic changes in pupillary diameter withrespect to a significant event in the experi-mental trial, however, ensures that generalfactors cannot affect the task-evoked pupil-lary response, except as they may system-atically vary during the course of an exper-imental trial. Nonetheless, there has beensome concern that a portion of the varianceof the task-evoked pupillary response maybe attributed to noncognitive variables, par-ticularly to the light reflex and to emotionalprocesses (Goldwater, 1972).

The possibility that the light reflex mightaffect the task-evoked pupillary responsemust be considered whenever subjects arepermitted to shift their gaze across a non-uniform visual field, such as a photograph(Hess, 1975). The controlled use of a fixa-tion point and the use of constant illumi-nation visual displays, however, render in-terpretations of phasic changes as reflectionsof the light reflex highly unlikely. Visual andoculomotor factors may be reasonably dis-counted when subjects are required to main-tain fixation and all experimental stimuli areaurally presented.

The question of emotional factors exertingsystematic influence on the phasic task-

evoked pupillary responses is more complex,particularly as such factors are often onlypoorly and intuitively defined. In the contextof the cognitive experiments described above,emotional involvement might be expected to

MULTIPLICAND MULTIPLIER

lzMEDIUM PROBLEM DIFFICULTY

4 6

TIME (SECSI

TIME (SECSI

Figure 9. Task-evoked pupillary responses for correctlysolved problems at three levels of difficulty for subjectsof high and low psychometrically defined intelligence.[At all difficulty levels, larger pupillary responses wereobserved for subjects in the lower intelligence group.(From "Pupillary responses during information pro-cessing vary with Scholastic Aptitude Test scores" byS. Ahern & J. Beatty, Science, 1979, 205, 1289-1292.Copyright 1979 by the American Association for theAdvancement of Science. Reprinted by permission.)]

288 JACKSON BEATTY

manifest itself in terms of task-induced anx-iety reactions. Several lines of reasoning tendto discount such arguments.

First, task-evoked pupillary responses havebeen reliably observed in tasks in which itis difficult to hypothesize emotional involve-ment. For example, in an auditory selectiveattention task (Beatty, Note 4), small butconsistent task-evoked pupillary responses ofapproximately .015 mm were observed fol-lowing presentation of nonsignal tone on theattended channel, whereas no responses werepresent to tones on the unattended channel.Because the event rate in that experimentwas 3 per sec, an average of 1.5 dilationswas obtained each second for the durationof the testing procedure (approximately 15min). Considering these dilations to be along string of stimulus selective, high-speedemotional reactions strains the concept ofemotional response to a point of absurdity.Similar arguments might be made for a va-riety of other simple cognitive tasks thatwould not appear to arouse emotion or toinduce anxiety for any subject.

A second reason to reject an emotion hy-pothesis as an explanation of task-evokedpupillary responses is based on an investi-gation of individual differences in pupillaryresponse amplitude. Ahern (Ahern, 1978;Ahern & Beatty, 1979, 1981) obtained pu-pillary responses in 39 university undergrad-uates tested in four cognitive tasks. Therewas a significant correlation between a psy-chometric measure of subject intelligenceand the amplitude of the task-evoked pupil-lary responses in the cognitive tasks. Therewas no significant correlation, however, be-tween the amplitude of the pupillary re-sponse and either state or trait anxiety(Spielberger, 1968). Differences in ampli-tude of the task-evoked pupillary responsebetween individuals appears to be a functionof differences in cognitive ability rather thanemotionality.

Third, in his study of information overloadin the digit span task, Peavler (1974) alsoaddressed the question of interpreting pu-pillary dilation as an indication of emotionalfactors. Peavler reasoned that his data areincompatible with any interpretation of thetask-evoked pupillary response as a reflec-

tion of task anxiety or other emotional re-sponses to the testing situation. If the task-evoked pupillary response reflected emo-tional factors due to fear of performancefailure, then a large dilation should accom-pany the presentation of the later digits inthe 13 digit strings because only at this timecould the subjects know that the limits ofcapacity would be exceeded and that theirperformance could not be perfect. No suchdilations to information overload were ob-served.

These lines of argument suggest thatemotional factors are relatively unimportantas determinants of the pupillary responsesobserved in carefully controlled information-processing tasks. Although emotional factorsare well known for their expression in theautonomic nervous system, the effects ofemotional arousal are generally longer last-ing than the brief phasic responses evokedby cognitive activity (Lang, Rice, & Stern-bach, 1972). Thus, changes in emotionalityare more likely to affect the tonic or basalpupillary diameter and not the phasic re-sponses under discussion here.

Several other types of potentially con-founding variables have also been tested.The effects of motivation on the pupillaryresponse were tested by Kahneman, Peavler,and Onuska (1968, Experiment 2) in a short-term memory task by varying monetary in-centives associated with correct performanceon different trials. Increasing the incentiveshad no effect on performance, nor did it af-fect the task-evoked pupillary response dur-ing the performance of either of the short-term memory tasks.

Clark and Johnson (1970) tested the pos-sibility that the task-evoked pupillary re-sponse in short-term memory experimentsmight come from the subject's knowledgeabout the results of previous pupillometricstudies and the demand characteristics of theexperiment. Varying these expectations hadno effect on the pupillary response, whichconformed to the pattern previously reportedby Kahneman and Beatty (1966).

Taken together, these lines of evidencelend support to the original hypothesis ofKahneman and Beatty (1966) that the task-evoked pupillary response reflects the mo-

PUPILLARY RESPONSES AND PROCESSING LOAD 289

mentary level of processing load and is notan artifact of noncognitive confounding fac-tors.

Processing Load and Processing Resources

Kahneman, in his 1973 theory, identifiedprocessing load with amount of attention or"mental effort" allocated to the performanceof a mental operation or task. This type ofcapacity theory, like that previously sug-gested by Moray (1967), proposed that morethan one mental operation might be simul-taneously executed providing that the jointdemand for attention does not exceed theavailable supply of attention or processingcapacity. The only restriction placed on thisprediction was that the two operations donot require the simultaneous use of fixedprocessing structures, such as sensory ormotor channels.

Performance data obtained with time-shared mental tasks, however, provide littlesupport for any general capacity model. Ap-parently trivial changes in the structure ofa task may produce large differences in itsinteraction with other tasks, even though theinformation-processing characteristics of thetask remain unaltered. Further, when taskdifficulty levels are varied, some pairs oftasks show performance interactions, whereasothers do not (Wickens, 1979). These find-ings have led most theorists toward multiplecapacity models (Navon & Gopher, 1979,1980; Sanders, 1979; Wickens, 1979, 1980).These models postulate several types of pro-cessing capacity that may be allocated amongmental operations. In such formulations,mental operations may be performed simul-taneously without interference if the demandfor capacity from any of the multiple poolsof capacity does not exceed the capacityavailable in each pool. These separate, qual-itatively distinct types of information-pro-cessing capacity are commonly called pro-cessing resources (Norman & Bobrow,1975). In a multiple resource model, pro-cessing load reflects the aggregate-process-ing resources consumed in the performanceof a mental operation.

The major issue confronting multiple re-source theory is the identification and spec-

ification of these specialized processing re-sources. This had proved exceedingly dif-ficult. Although it has been proposed thatresources may be identified from task inter-actions alone (Navon & Gopher, 1979),most theorists have sought other sources ofconverging information in attempting toidentify these specific processing capacities.

Kinsbourne (Kinsbourne & Hicks, 1978),for example, suggested that spatially re-stricted, functionally specialized regions ofthe cerebral cortex constitute the processingresources of multiple resource theory. Kins-bourne and his collaborators presented asubstantial amount of evidence suggestingthat the interference between two mentaloperations increases as a function of the spa-tial proximity of the primary cortical regionsinvolved in performing those operations. Arelated view has been proposed by Wickens(1980), who suggested that processing re-sources may be categorized by input mo-dality (visual or auditory), hemispheric pro-cessing specialization (spatial or verbal), andtype of responding (vocal or manual). Re-stated in terms of cortical regions, the au-ditory and visual sensory cortices, the asso-ciation cortices of the right and lefthemisphere, and the highly differentiatedhand and mouth-respiratory regions of themotor and premotor cortex constitute pu-tative processing resources in Wickens'smodel. The view that the multiple, function-ally specialized processing resources of thehuman information-processing system maybe identified with functionally specializedregions of the human cerebral cortex is asensible suggestion.

Nevertheless, if restricted regions of thecerebral cortex in fact form the resources ofthe human information-processing system,why should the task-evoked pupillary re-sponse reflect their utilization? The answerseems to lie in the dynamic interaction be-tween cerebral cortex and the reticular ac-tivating system of the brainstem, coupledwith the fact that pupillary movements pro-vide a sensitive indicator of reticular func-tion. Luria provided a very clear overviewof the reciprocal interactions of cerebral cor-tex and the reticular core in his popular 1973monograph:

290 JACKSON BEATTY

[The] maintenance of the optimal level of cortical toneis essential for the organized course of mental activity.. . . [However], the structures maintaining and regu-lating cortical tone do not lie in the cortex itself, butbelow it [in the reticular formation of the brainstem],. . . Some of the fibres of [the] reticular formation runupwards to terminate in higher nervous structures suchas the thalamus, caudate body, archicortex and, finally,the structures of the neocortex. . , . [They play] a de-cisive role in activating the cortex and regulating thestate of its activity, (pp. 45-46)

The higher levels of the cortex, participating directlyin the formation of intentions and plans, recruit thelower systems of the reticular formation of the thalamusand brain stem, thereby modulating their work andmaking possible the most complex forms of consciousactivity, (p. 60)

Task-evoked pupillary dilations very likelyreflect the cortical modulation of the retic-ular core during cognitive processing. Thepupillary system, like other peripheral sys-tems, receives input from most structures inthe reticular formation. It must be remem-bered that the efferent fibers leaving retic-ular structures typically bifurcate, sendingone branch upward to the forebrain and an-other downward, synapsing on a wide varietyof motor nuclei (Brodal, 1981). Thus, anyresponse to forebrain commands modulat-ing activity in the cortico-reticular re-ticulo-cortical loop will also make its effectsfelt in the autonomic periphery. For this rea-son, pupillary movements have served neu-rophysiology well as a sensitive indicator ofreticular system discharge (Moruzzi, 1972).

The evidence reviewed above argues thatthe amplitude of task-evoked pupillary re-sponses provides a reliable index of task-in-duced processing load. The use of time-locked averaging methods ensures that onlychanges related in time to the mental op-erations under study are measured. Theseresponses grow larger as task parameters arevaried to increase task demands for pro-cessing resources (Navon & Gopher, 1979).Moreover, the measure reflects variations inprocessing load between qualitatively differ-ent mental operations in a reasonable andconsistent manner. For this reason, task-evoked pupillary responses may provide aglobal indication of task-induced processingload even when the composition of process-ing resources differs between tasks. Thereis nothing incompatible in viewing the pu-pillary response as a measure of the aggre-

gate task-induced utilization of multiple pro-cessing resource. This idea is in some waysanalogous to the use of a general phys-iological measure such as oxygen uptake asan indicator of the aggregate metabolic de-mands of a set of functionally distinct or-gans.

The task-evoked pupillary response mayalso provide an indication of the joint de-mand for resources in pairs of time-sharedtasks. No experiments explicitly testing thistwo-task prediction, however, have yet beenpublished. Nevertheless, some data obtainedby Kahneman, Peavler, and Onuska (1968,Experiment 1) on the effect of motor re-sponding and cognitive processing are rele-vant here. Kahneman et al. examined theeffects of verbalization on the task-evokedpupillary response in a short-term memorytask at two levels of difficulty. Subjects lis-tened to a string of four digits that they wereto repeat or transform by adding one(Kahneman & Beatty, 1966). After presen-tation, they either repeated the appropriateresponse twice at.the rate of 1 digit per secor mentally produced the response in thefirst interval and verbally produced it in thesecond. The more difficult digit transfor-mation task yielded systematically largerpupillary dilations regardless of verbaliza-tion condition. The form of the response wasunaltered in the absence of verbalization.The effect of verbalization was to increasethe amplitude of the task-evoked pupillaryresponse. These data are consistent with thehypothesis that the organization of an overtmotor act places additional demands on in-formation-processing resources that are re-flected in the task-evoked pupillary response.This finding indicates that a physiologicalapproach to the study of dual task interac-tions is possible.

Finally, it is interesting to note that, al-though pupillometric data formed the basisfor Kahneman's general capacity theory, intheir original research reports Kahnemanand Beatty (1967) proposed a concept ofprocessing load suggestive of modern mul-tiple resource theories:

The frequent use of the concept of processing load inthe present paper has been guided by a simple analogy:consider a houseful of electrical devices that are var-iously put in operation by manual switches or by their

PUPILLARY RESPONSES AND PROCESSING LOAD 291

own internal governor systems. The total amperage de-manded by the entire system at any one time may easilybe read on an appropriate electrical instrument outsidethe house. Processing load is here construed as analo-gous to an aggregate demand for power, and there isground for the hope that the pupil may function as auseful approximation to the relevant measuring device,(p. 104)

The task-evoked pupillary response doesserve as such a measure. It provides a reli-able and sensitive indication of within-taskvariations in processing load. It generates areasonable and orderly index of between-task variations in processing load. It reflectsdifferences in processing load between in-dividuals who differ in psychometric abilitywhen performing the same objective task.For these reasons, the task-evoked pupillaryresponse provides a powerful analytic toolfor the experimental study of processing loadand the structure of processing resources.

Reference Notes1. Beatty, J., & Schluroff, M. Pupillometric signs of

brain activation reflect both syntactic and semanticfactors in language processing. Paper presented atthe meeting of the Society for PsychophysiologicalResearch, Vancouver, October 1980.

2. Beatty, J., & Wagoner, B. L. Activation and signaldetection: A pupillometric analysis (Tech. Rep. 10).Los Angeles: University of California, Los Angeles,Human Neurophysiology Laboratory, August 1977,

3. Beatty, J., & Parasuraman, R. Task-evoked pupil-lary responses in auditory signal detection. Unpub-lished manuscript, 1981. (Available from J. Beatty,Department of Psychology, University of California,Los Angeles, Los Angeles, California 90024).

4. Beatty, J. Pupillometric signs of selective attentionin man (Tech. Rep. No. 18). Los Angeles: Universityof California, Los Angeles, Human NeurophysiologyLaboratory, February 1980.

5. Wickens, C. D. The effects of time sharing on theperformance of information processing tasks: Afeedback control analysis (Tech. Rep. No. 51). AnnArbor: University of Michigan, Human PerformanceCenter, 1974.

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